How much help would a friendly Angola and Mozambique be to apartheid South Africa and Rhodesia
Ian Smith, the Prime Minister of Rhodesia during the Bush War, said that if Salazar, the head of Portugal's government during the Estado Novo, had lived another 10 years then Rhodesia would have survived. By that he meant that Salazar's extended life would have preserved Portuguese colonial rule in Angola and Mozambique, the latter of which was host to anti-Rhodesian rebels and allowed them to conduct cross-border raids against Rhodesian targets.
As this map shows, with Mozambique remaining Portuguese Rhodesia's strategic situation is vastly improved compared to OTL. South Africa would also not have to worry about the war in Angola, which as has already been stated was a serious issue and probably expedited the fall of Apartheid.
In regards to the OP I think the answer depends on who the backer is. Obviously, all major communist countries are out. The UK's policy positions in Southern Africa were what created Rhodesia in the first place, so I don't see them being the backer without altering the situation entirely. If we're sticking with generally OTL countries, then France could be an option. IIRC France was one of the only countries besides Portugal and South Africa to abstain or vote against the UN motion sanctioning Rhodesia. Perhaps if France was a bit more committed to its own colonial empire in Africa this could result in increased support of the other colonial/white governments.
Alternate powers which could support SA/Rhodesia would include: A Japan that wins/doesn't fight in WW2 and looks towards Africa for more resources. This isn't that far fetched given the cooperation between Japan and South Africa in OTL. An alternate China, perhaps one where the Communists are defeated and the Nationalists are looking to expand their own sphere of influence, could be a patron to SA/Rhodesia.